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Appendix II.


being gradually brought into gear with the wheel. Usually the
wheel is not turned round automatically, and then the hob has to
•do the double duty of rotating the wheel and cutting the teeth.
The fault of this method is precisely that of stocks and dies in
•screw-cutting, for the worm thread has a different angle at the top



to what it has at the bottom. Better results are obtained by
using a machine like a short lathe, and mounting the wheel on
a vertical shaft passing through a fixed saddle, while the hob is
-driven between the lathe centres. The wheel shaft is then rotated
at the proper velocity regarding the worm, by means of change
"wheels.                          »